Bipolar ROM questions...

ifkz

Well-known member
Joined
Oct 3, 2006
Messages
1,544
Reaction score
45
Location
Texas
I think one or two of my project boards have problems with bipolar roms, but I am not sure. I believe bipolar eproms are mainly used for storing sprite & background color data...please correct me if I am wrong.

1. Is there a bipolar roms for beginners document somewhere?
2. Are these re-writable?
3. What is a common indicator of a bipolar rom versus a mask rom versus a memory chip? They all have speed ratings, does it all come down to the stamped codes on each chip? Do they come in specific packages....do all bipolar eproms look like standard eprom packages?
 
Bipolar PROMs are fuse link programmable. You program them by burning a fuse inside them. They are not reusable as you can only open more fuses and cannot reset any blown ones. These are old school and come mainly in DIP packaging - either ceramic or plastic and do not have windows on them.

Mask ROMs are programmed at the factory during the time of the chip being manufacturered. This is why they are called Mask ROMs. They use a metal mask to control the deposits on the chip when it's being fabricated. This type of chip is not reprogrammable at all. These generally come in DIP format and have no windows on them.

OTP ROMs are EPROMs without windows. You can program them once, hence the name One Time Programmable ROM. They cannot be erased unless you want to try using an x-ray machine on them. These are available in DIP, PLCC, SOIC, and other formats and have no windows on them.

EPROMs are Erasable Programmable ROMs. They come in ceramic bodies in DIP and PLCC packaging and have those clear windows on them. They can be erased with ultraviolet light and reprogrammed with a special programmer. Beware that they have varying requirements for programming voltages. Too low and it won't program. Too high and you blow the chip.
 
This answers most of my bipolar prom questions, thanks.
 
Bipolars are slim DIP chips, same width as TTL chips. Their benefit is they are very high speed but they have very low capacity compared to normal eproms, often only used for lookup tables, often in the colour section of boards.

When people talk about PROMs they usually mean Bipolar PROMs, even tho a PROM technically is an OTPROM as it is programable but not erasable. The "one time" prefix is fairly superfluous if erasing is not an option.

The OTPROM (one time programable ROM) is the same chip internally as a normal EPROM, without the window. The window was actually a fairly expensive part of the chip so ditching that made things much cheaper. If you could somehow drill a hole in the top of the chip without wrecking the inards you could in erase it and re-use it.

What is the point of all these types? EPROM, OTPROM, Mask ROM?

Money basically, specifically saving it!

On boards the executable code is usually contained in EPROMs so if there were any bugs they could erase the chips and program with the new code without throwing any expensive parts in the bin. The price of EPROMs when these boards were made was very high, easily 25% of the price of the entire board. Once they were sure the game code was not going to change then they could save money by using OTPROMs in place of the EPROMs. Its very common to see boards with a mix of EPROMs and OTPROMs and there is a high chance that it is the EPROMs that contain the actual program code.

Mask ROMs are chips that are fabricated with the data built into the structure of the chip, these were commonly used from the very late 80s onwards for graphics and sound sample data - data they knew was 100% from day one and was never going to change.

On earlier boards you often see a mix of EPROMS and OTPROMs instead of the masks, the reasoning is the same - gfx and audio data that they knew was 100% correct so there was no need to spend money on chips that could be erased.

Mask ROMs solve the same problem but there was scope to save even more money. Of course setting up the manufacture of mask ROMs was expensive but if they were going to make enough the cost per unit would fall below that of using EPROMs or PROMs, especially for the very high capacity chips when the price of multi megabyte chips was over $100. An added bonus was that they could design whatever pinout they wanted. If the board was never going to use the /CE pin then that could be done away with, no need for a pin to take a programming voltage as the chip is never programmed, so that pin can go too. The result was they could have a chip with fewer legs and still have the same capacity, this is why some mask ROMs do not have a direct drop in EPROM equivalent. If you can shave a inch off the PCB size by having smaller chips then that in itself makes for a massive saving in manufacturing cost is you are making thousands of boards.

Of course once the masks were made any mistakes in the data were locked in and aside from throwing them away and making new ones they had no option but to live with it. I imagine the lead artist on Shadow Warriors got a roasting when it was discovered that the main characters socks changed colour between when he was walking or fighting. If that data was in mask ROMs then it would have cost hundreds of thousands of dollars to fix, which is probably why the "bug" is there to this day.

There is also the EEPROM - the electrically erasable programable ROM which doesnt need to be erased with UV light. Not seen much on arcade boards tho.
 
Last edited:
Bipolars are slim DIP chips, same width as TTL chips. Their benefit is they are very high speed but they have very low capacity compared to normal eproms, often only used for lookup tables, often in the colour section of boards.

Not always!!!

I have PROMs that go up to 4k x 8 and are as wide as a standard EPROM.

They are programmed by blowing fuses just the same as the 32 x 8bit tiny ones and are very high speed devices when compared to EPROMs.
 
Hi,
I saw somewhere in the net a small pcb.It plugs into the socket of the PROM, on the pcb is a 2716 EPROM which contains the data of the PROM. I mean the description is on a webside for old Gottlieb SYSTEM 1 pinballs.
Greets to All
 
Hi,
I saw somewhere in the net a small pcb.It plugs into the socket of the PROM, on the pcb is a 2716 EPROM which contains the data of the PROM. I mean the description is on a webside for old Gottlieb SYSTEM 1 pinballs.
Greets to All

Yes,

The only problem is that the 2716 is SLOOOOOOW... on the order of 350 to 600 ns in cycle times. Bipolar PROMs are on the order of 20 to 70ns in speed for their cycle times.

The 2716 replacement will not always work.
 
Yes,

The only problem is that the 2716 is SLOOOOOOW... on the order of 350 to 600 ns in cycle times. Bipolar PROMs are on the order of 20 to 70ns in speed for their cycle times.

The 2716 replacement will not always work.

Yep. All depends on the application though. Pinballs, particularly early ones like the Gottlieb in question, generally are ok with EPROMs.

Should the situation work for it, you could also make up something with larger EPROM's, such as a 2764. Can get those in the 120ns-150ns speed range. All depends on the application.

-Hans
 
I think the System 1 CPU (PPS-4) runs at like 200kHz (0.2 MHz). Even it managed to read the PROM *EVERY* clock cycle, there would be 5000ns between reads. There would seem to be plenty of margin with an EPROM in this case.

In the vector generator of an Atari game, or a color PROM for a raster game, the required speed might justify the speed of a bipolar PROM. But for a Gottlieb Sys 1... not so much.
 
Back
Top Bottom